The present invention relates to a positive displacement machine having a particular stroke characteristic, and namely to vane type positive displacement machine which can operate either as a pump or as a motor to convert energy by means of stream of fluid medium.
Vane-type positive displacement machines are known which include essentially the following parts:
a rotary shaft for exchanging mechanical energy;
a cylindrical rotor fixedly mounted on the shaft;
a housing surrounding the rotor, the housing including flanges directed at right angles to the axis of rotation of the rotor, and a tubular body (stator) defining a cylindrical inner surface which is offset relative to the axis of rotation of the rotor;
a plurality of vanes slidably guided in slots of the rotor and engaging the inner surface of the stator in such a manner as to delimit a plurality of variable volume working chambers; and
control parts or valves for controlling the intake and discharge of the fluid.
A machine of this kind is described in French Pat. No. 2,203,421, whose particular feature is the shape of the inner surface of the tubular housing. The closed contour line of this inner surface is in the form of a hypertrochoid which can be in theory mechanically generated (a NC machine is in theory not necessary) and can be described by the following complex equation: ##EQU1##
wherein j is imaginary unit, and exp j is imaginary exponential function;
A.sub.k,.alpha..sub.k,.beta..sub.k are real numbers defining parameters of a particular form of the hypertrochoid;
K is a real parameter varying between zero and a particular value K*, where the affix once covers the hypertrochoid;
n is an integer defining the order of the hypertrochoid.
The disadvantage of this known embodiment is the difficulty in providing a sufficient seal between the pressure side and the adjoining suction side between the tubular part of the housing (stator) and the rotor, caused by the fact that the hypertrochoid cannot conform the profile of the rotor over a finite center point angle.
Attempts have already been made to avoid this disadvantage by creating local curve sections which deviate from a hypertrochoid. For instance, (a) the contour line of the stator surface is provided with a circular arc having a smallest possible clearance relative to the profile of the rotor, and conforming this profile over a small angle sufficient for guaranteeing a sufficient sealing action; (b) this circular arc is connected to the rest of the hypertrochoidal contour line of the stator surface by another arc having mostly a form of a circular segment whose center point is evidently offset with respect to the center of the rotor.
This solution has a serious drawback resulting from very disadvantageous development of the curvature along the unavoidable connection arc in the case when the order of symmetry of the hypertrochoid is larger than one. In such a design of the machine the sliding vane has a tendency to retract into the rotor exactly at a point where it is expected to slide out as fast as possible. It has been found that the provision of the before described connection arc becomes even more disadvantageous when the angle formed by the sealing arc is increased. This case occurs particularly in such constructions of positive displacement machines where the sealing vanes are replaced by rollers.